The Big Muff Pi is a monster fuzz pedal that swallows the incoming signal and spits out a creamy deep fuzz tone that's instantly recognizable.
For single note leads the BMP provides a thick full sound loaded with singing sustain and warm feedback.
For rhythms the BMP offers massive rich fuzz that works well for simple chord voicings and riffs. It is, however, a little limited as its thick sound masks more subtle voicings.
While the BMP may not be the best fit for every situation; it is a great go to effect to push things over the edge.